The end compound of the present invention, i.e. N-carbobenzoxy-L-aspartic anhydride, is an important intermediate for peptide synthesis. For instance, this compound is reacted with a lower alkyl ester of L-phenylalanine, and by eliminating the protecting group (i.e. carbobenzoxy group) through hydrogenolysis, a lower alkyl ester of .alpha.-L-aspartyl-L-phenylalanine is produced. This peptide has the sweetness of sucrose and has a great potential for use as a new type of sweetener.
By dissolving or suspending N-carbobenzoxy-L-aspartic acid in a solvent and letting a dehydrating agent act on the resulting solution or suspension, N-carbobenzoxy-L-aspartic anhydride is produced as a solution or suspension. For commercial production, this solution or suspension is desirably reacted with a lower alkyl ester of L-phenylalanine without isolating the N-carbobenzoxy-L-aspartic anhydride, and for this reason, acetic anhydride which does not produce any by-product that may have adverse effects on the subsequent steps is preferably used as the dehydrating agent. The presence of a large amount of residual acetic anhydride in the reaction liquor should be avoided because this adversely affects the subsequent steps. Therefore, the proper amount of the acetic anhydride used as the dehydrating agent ranges from 0.7 to 1.3 mols per mol of the starting material N-carbobenzoxy-L-aspartic acid. To minimize the undesired conversion of the end product into a racemic form, the reaction temperature ranges from -10.degree. to 100.degree. C., preferably from 0.degree. to 80.degree. C.